They've actually toyed with this idea in the toys on a couple of occassions, or at least something similar.

There were the Micromaster Transports, which included the illustrious Erector, who were micromasters included with a larger trailer (or aircraft carrier) that formed a transport or base they could interact with in robot mode.

There were also the Mega Pretenders, like Thunderwing, whose Pretender shells also transformed, and who they could latch onto in their alternate mode for a rudimentary supermode. He was basically a starfighter.

There were also Action Masters who interacted with exo-suits, and unreleased action master exo-suits which would have given them a car alternate mode, or something similar to it.

Even the recent Cyberverse line included small figures with playset vehicles or bases they could ride and interact with, is this kind of the same thing you were thinking of?

I'm always down for a new gimmick that changes how a toy can be played with, and I liked the micromaster transports and the ultra pretenders.